Recently, portable equipment which uses a battery, such as a mobile phone, a digital camera and so on, has been widely developed. Such portable equipment commonly uses a constant voltage circuit which supplies a constant power supply voltage. The constant voltage circuit widely employs a voltage step-up/step-down switching regulator to obtain a regulated voltage.
A conventional voltage step-up/step-down switching regulator employs two DC-DC converters, i.e., a step-up converter and a step-down DC-DC converter. The conventional voltage step-up/step-down switching regulator performs a step-down operation by activating the step-down DC-DC converter when an input voltage is larger than an output voltage so as to output a predetermined constant voltage. Meanwhile, the conventional voltage step-up/step-down switching regulator performs a step-up operation by activating the step-up DC-DC converter when the input voltage is smaller than the output voltage.
FIG. 1 illustrates a conventional voltage step-up/step-down switching regulator 100. FIG. 2 illustrates a timing chart of waveforms of each part of the circuit of FIG. 1.
The voltage step-up/step-down switching regulator 100 includes an oscillator 110, an operational amplifier 116, PWM (pulse width modulation) comparators 112 and 114, and resistors R101 and R102.
A signal A is an output signal from an error amplifier (not shown) and is an amplified signal of a difference voltage between a reference voltage and a proportional-voltage proportional to the output voltage. The signal A is defined as an error signal A. A signal B is an upper peak voltage V2 and a signal C is a low voltage V1 which is lower than the upper peak voltage V2.
The oscillator 110 generates a triangular wave G which swings between the upper peak voltage V2 as an upper limit voltage and the lower peak voltage V1 as a lower limit voltage.
The operational amplifier 116 forms an inverting amplifier to have a reference voltage with the upper limit voltage V2 of the triangular wave. If the resistors R101 and R102 have equal resistance value from each other, a signal D becomes an waveform equivalent to an inverted signal of the error signal A with respect to the upper limit voltage V2.
The error signal A is input to an input of the PWM comparator 112. The triangular wave G, which is an output of the oscillator 110, is input to another input of the PWM comparator 112. The signal D, which is the output signal of the operational amplifier 116, is input to an input of the PWM comparator 114. Similarly to the PWM comparator 112, the triangular wave G is input to another input of the PWM comparator 114.
The voltage step-up/step-down switching regulator 100 further includes NMOS transistors S101 and S102, diodes D101 and D102, a coil L101 and a capacitor C101.
When the voltage step-up/step-down switching regulator 100 performs the step-down operation, the NMOS transistor S102 is off and the NMOS transistor S101 only performs on/off operation. If an input voltage Vin becomes larger than an output voltage Vout, an on-time of the NMOS transistor S101 becomes shorter. If input voltage Vin becomes closer to the output voltage Vout, an on-time of the NMOS transistor S101 becomes longer.
When the voltage step-up/step-down switching regulator 100 performs the step-up operation, the NMOS transistor S101 is on and the NMOS transistor S102 only performs on/off operation. If the input voltage Vin becomes smaller than the output voltage Vout, an on-time of the NMOS transistor S102 becomes longer. If the input voltage Vin becomes closer to the output voltage Vout, an on-time of the NMOS transistor S102 becomes shorter.
The PWM comparator 112 compares the voltage of the error signal A with the voltage of the triangular wave G. If the voltage of the error signal A is larger than the voltage of the triangular wave G, the PWM comparator 112 outputs a signal F with a high level. If the voltage of the error signal A is smaller than the voltage of the triangular wave G, the PWM comparator 112 outputs the signal F with a low level.
The PWM comparator 114 compares the output voltage D of the operational amplifier 116 with the voltage of the triangular wave G. If the voltage of the triangular wave G is larger than the output voltage D, the PWM comparator 114 outputs a signal E with a high level. If the voltage of the triangular wave G is smaller than the output voltage D, the PWM comparator 114 outputs a signal E with a high level.
When the error signal A is within the voltage range of the triangular wave G, the PWM comparator 112 outputs a signal F with a pulse wave and performs the step-down operation by making the NMOS transistor S101 on/off. The output voltage D of the operational amplifier 116 exceeds the upper limit voltage V2 of the triangular wave G during this time period. As a result, the output signal E of the PWM comparator 114 becomes low level and the NMOS transistor S102 becomes off.
When the error signal A exceeds the upper limit voltage V2 of the triangular wave G, the output signal F of the PWM comparator 112 becomes high level and the NMOS transistor S101 becomes on. Meanwhile, when the output voltage D of the operational amplifier 116 is within the voltage range of the triangular wave G, the PWM comparator 114 outputs a signal E with pulse wave and performs the step-up operation by making on/off with the NMOS transistor S102.
Thus, the voltage step-up/step-down switching regulator 100 controls the output voltage to obtain a predetermined constant voltage by exchanging the operational mode between step-up and step-down modes in accordance with the input voltage Vin. However, the conventional voltage step-up/step-down switching regulator 100 needs two PWM comparators, i.e., the PWM comparator 112 to control the step-down switching element of the NMOS transistor S101 and the PWM comparator 114 to control the step-up switching element of the NMOS transistor S102.
Further, in addition to the error amplifier which is commonly used, the operational amplifier 116 is needed to invert the error signal A with respect to the upper limit voltage V2 and to input the inverted error signal to the PWM comparator 114 for the step-up operation. Furthermore, the conventional voltage step-up/step-down switching regulator 100 has a cost penalty because the PWM comparators and the operational amplifier are generally formed of an analog circuit which needs many circuit elements and requires the circuit elements to have high precision.